THE EVOLUTION THEORY 123 



left half, according to the species, is distinguished by a colour 

 ranging from light to white, and by the absence of the eye. 

 Both eyes are fixed close together on the upper side which is 

 distinguished by varied design, and frequently by bright colours. 

 Yet in spite of the pronounced asymmetry the existing flat- 

 fishes have doubtless descended from symmetrical ancestors, 

 for each individual must to-day pass in its own body through all 

 the stages of its phylogeny. 



The little flat-fish is hatched, having a lateral-symmetrical 

 structure, being in nowise distinguished from the embryos of 

 other fishes, excepting its destiny. But with increasing growth 

 it exhibits increasing differences. Of the eyes, which were 

 originally fixed one on each side, one leaves its normal position 

 and wanders to the other side. Simultaneously the flattening 

 of the body proceeds until at last the final stage of the adult 

 animal is reached (fig. 34). 



To their structure corre- 

 sponds their mode of life. Their 

 domicile is the sea-floor. Here fjssMl fV 

 they lie with the light under- 

 side on the sand, often buried 

 in the sand. They are not 1 2 



easily detected, for the resem- FIG. 34. SHIFTING OF THE EYES DUR- 

 blance of the upper side, which ING THE DEVE p ^ F A YOUNG 



is alone visible on the Ocean- ( i } Original position ; (2) second stage ; 

 bed, equally well protects them (3) final position (after Leunis-Ludwig). 



against their numerous enemies 



and conceals them from their unsuspecting prey. 



One of the best instances of adaptation to special conditions 

 of life is furnished by the Baleen whales, the giants of modern 

 times. Only the ocean with its inexhaustible wealth of life is 

 able to produce and maintain such monsters. It is impossible 

 to conceive the volume of food which must be taken every day 

 by an animal weighing more than 50,000 kilogrammes in order 

 to exist. The problem becomes all the more difficult when we 

 learn that owing to the construction of the pharynx they are 

 dependent for food on small animals, minute transparent snails, 

 cuttle-fishes, shrimps and prawns, and small fishes not larger 

 than a herring. If these giants had to capture their prey 



